The present invention is concerned generally with a method, separation medium and apparatus for selectively extracting alkaline earth cations from acidic aqueous solutions. More particularly, the invention is concerned with a free-flowing separation medium of a Crown ether dispersed onto an inert substrate material, a method for using that separation medium and an apparatus for selectively extracting alkaline earth cations, and particularly strontium cations, from an acidic aqueous sample.
Growing public health and safety concerns over the use of nuclear technology, both in the production of power and in the manufacture of nuclear weapons, have made increasingly important reliable methods for the determination of various radiostopes in environmental and biological samples. The beta emitting isotope, 90Sr, produced by nuclear fission, is of particular concern. Because of its chemical similarity to calcium, 90Sr released to the environment can quickly enter the food chain. Ultimately, the 90Sr can be permanently incorporated into the inorganic matrix of human bones. There is, therefore, a clear need for an analytical procedure and methodology suitable for use in the routine monitoring of persons whose activities expose them to the risk of internal contamination from this element and for the determination of the levels of radiostrontium in various environmental samples (e.g., soils, plants, natural waters, and waste streams).
Numerous prior methods have been described for performing such determinations. An essential feature of most of these methods is the separation and purification of the strontium, both to remove radioisotopes that can interfere with subsequent beta counting and to free it from the large quantities of inactive substances typically present, particularly calcium.
Typical past methods of separation of Ca and 90Sr have relied upon the greater solubility of calcium nitrate in strong nitric acid solutions. This approach ultimately provides satisfactory results but requires numerous process steps, and repeated precipitations must be performed in order to achieve substantially complete recovery of 90Sr. Other methods of separation involve precipitation of strontium sulfate or strontium rhodizonate, sorption of strontium on an cation exchange resin from a solution of a chelating agent such as CYDTA or EDTA, and liquid-liquid extraction with organophosphorous acids. These methods, however, suffer from serious shortcomings. In the case of the precipitation and extraction methods, the extracted strontium is contaminated with significant amounts of calcium. Cation exchange processes require precise control of pH value due to the narrow range of pH necessary to achieve adequate separation. Moreover, the acceptable pH extraction range is affected by the concentration of calcium present that gives rise to different pH value ranges needed for effectively treating different waste samples.
In the recent past, attempts have been made to use macrocyclic polyethers (Crown ethers) to separate strontium and calcium. Such work has involved a chloroform solution of dicyclohexano-18-Crown-6 to separate strontium traces from macroscopic amounts of calcium cations. This method has also been applied to the determination of the relative amounts of 90Sr and 89Sr in milk. In another related process, a combined procedure involving cation-exchange/extraction/precipitation has been developed that incorporates the extraction of strontium picrate into a methylene chloride solution of dicyclohexano-18-Crown-6 to isolate traces of strontium from milk and drinking water.
In a related process involving chromatographic separation of alkaline earths, solutions are used consisting of various polyethers in chlorinated hydrocarbons dispersed onto siliconized kieselguhr as a stationary phase. Separation of calcium and strontium proceeds by use of either dibenzo-18-Crown-6 or dicyclohexano-18-Crown-6. Unfortunately, each of these Crown ether-based strontium extraction procedures is wholly ineffective for relatively acidic samples. This is a particularly troublesome shortcoming because it is often most desirable to digest biological or environmental samples or leach the raw sample using an acid to produce a feed solution for extraction, and such a feed solution is often several molar in acid.
Co-assigned U.S. Pat. Nos. 5,100,585, 5,110,474 and 5,346,618 by the present inventors teach the manufacture and use of a chromatographic medium for selectively separating strontium or technetium values from acidic compositions from various sources. The solid phase chromatographic medium made and used in those patents comprised a solution of a Crown ether dissolved in a diluent that was slightly soluble or insoluble in water, but capable of dissolving a substantial quantity of water such as octanol, which solution was itself dispersed onto a solid inert resin substrate material.
A few years after the filing of the applications that became the above U.S. patents, Benzi et al. (1992) J. Radioanal. Nucl. Chem., Letters, 164(4):211-220 reported on the use of 18-Crown-6 (18C6), dibenzo-18-Crown-6 (DB18C6) and 24-Crown-8 (24C8) as well as open chain ligands (podans) adsorbed on Amberlite(copyright) XAD-4 and XAD-7 resins or Kieselgel as supports for removal of radium cations from aqueous solutions. Those authors reported the supported crown ethers to be inefficient for that extraction, whereas the supported open chain ligands were said to provide satisfactory distribution coefficients for the radium removal.
The above-noted co-assigned patents provided a large technological advance over the liquid-liquid separation techniques that preceded them, and from which their technical advance grew. However, the separation medium of those patents exhibited changes upon elution of the captured strontium cations that minimized their usefulness for a subsequent separation, including loss of diluent to the effluent medium. Still further, the amount of strontium cation-extracting Crown ether present on any given support was limited because of the presence of the diluent.
It would therefore be beneficial to provide a method, separation medium and apparatus for separating alkaline earth cations such as strontium cations from acidic aqueous samples such as biological, commercial waste and environmental samples that do not exhibit the negative attributes of our prior technologies. The method, separation medium and apparatus of the present invention that are described hereinafter can overcome those negative attributes, while maintaining the previously achieved advances.
The present invention contemplates a separation medium, an apparatus for carrying out a separation such as a chromatographic column or cartridge containing the separation medium, and a method of using the separation medium to separate alkaline earth cations, such as barium cations from radium cations, and particularly strontium cations from other alkaline earth cations, present in an acidic aqueous solution. The separation medium comprises free-flowing particles having a di-4,4xe2x80x2(5xe2x80x2)[C3-C8-alkylcyclohexano]18-Crown-6, such as di-t-butylcyclohexano-18-Crown-6, that exhibits a partition ratio between n-octanol and 1 M nitric acid (DCrown=[CrownOrg]/[crown]Aq) of greater than about 103 and usually of about 103 to about 106 dispersed onto an inert, porous support such as polymeric resin or silica particles. The separation medium is free of a diluent, and particularly free of a diluent that is (i) insoluble or has limited (sparing) solubility in water and (ii) capable of dissolving a substantial quantity of water.
A separation apparatus for extracting alkaline earth cations from an acidic aqueous solution comprising the above separation medium in a support vessel is also contemplated. A contemplated apparatus has a fluid inlet and fluid outlet and one or more porous supports within the vessel for maintaining the separation medium in a desired position. A contemplated support vessel is typically glass or plastic such as polyethylene or polypropylene and is typically a chromatographic column or cartridge.
A method for separating alkaline earth cations such as strontium cations from an acidic aqueous sample is also contemplated. In accordance with this method, a free-flowing separation medium of a before-described di-4,4xe2x80x2(5xe2x80x2)[C3-C8-alkyl-cyclohexano]-18-Crown-6 such as di-t-butylcyclohexono-18-Crown-6 dispersed onto an inert, porous support such as a resin material is contacted with an aqueous acidic sample containing dissolved alkaline earth metal cations, such as strontium cations. The separation medium is free of a diluent as noted before. The contact is maintained for a time period sufficient for the alkaline earth cations to be extracted from the sample solution to the separation medium to form a solid phase-loaded separation medium and a liquid phase alkaline earth cation-depleted sample. The solid and liquid phases are thereafter separated.
In preferred practice, the alkaline earth cations are selectively eluted from the loaded separation medium by contacting the loaded separation medium with an aqueous solution having a pH value of about 1 or greater and a nitrate anion concentration of about 0.1 M or less. It is also preferred that the original contacting step be carried out in a before-described separation apparatus such as a chromatographic separation column or cartridge by passing the acidic aqueous sample into the separation apparatus to contact the separation medium therein, and that the elution of alkaline earth cations be carried out by passing an aqueous solution of pH 1 or greater through the loaded separation medium in the separation apparatus.
The present invention has several benefits and advantages.
One benefit of the invention is the separation medium that is free of previously used diluent that can contaminate an alkaline earth cation-containing effluent.
An advantage of the invention is an improved method and apparatus for extracting alkaline earth cations such as strontium cations dissolved in a strongly acidic solution.
Another benefit of one aspect of the invention is the higher loading of extractant Crown ether on the separation medium that can be achieved in the absence of diluent that leads to a greater capacity of the separation medium particles.
Another advantage of one aspect of the invention is that substantially greater partition ratios for barium can be achieved using the new separation medium, while maintaining a desired difference in partition ratios between barium and radium cations.
Still another benefit of the invention is that prolonged washing of the separation medium with 1 M nitric acid does not appear to affect the elution profile of subsequently loaded separation medium particles.
Still further benefits and advantages will be apparent to the skilled worker from the disclosure that follows.